The surface is clearly planar, suggesting to me a natural fracture, which has intersected 3-dimensional structures in the rock, which are consequently displayed as 2-dimensional patterns. That is, the patterns are not just on the surface; they represent solid structure.

In this sort of setting, my mind automatically thinks of concretions/nodules, which are often ovoid and concentrically banded. But your photo shows something more complex and interesting. There are aspects that suggest it is strain-related, ie. structural in nature rather than chemical.

But I don't actually know. It is great to see something new; thank you.

...Interesting idea; I certainly see where you're coming from (convection cells).

However, volcanic mud pools are unlikely in the Dorset geological setting, and would have been an unlilkely environment for an ammonite to live in (but maybe it was conned by reading that article about the benefits of mud baths in Health Spa Weekly, June 202,323,728BC)

Some types of origin seem unlikely because of the rectilinear nature of the 'cells' (various processes seem ruled out because of a lack of polygonal form). I also note that, while the exposed surface shows lines dividing the 'cells', the nature of the curvature of the laminae in the cells suggests that the exposed surface itself is another such dividing line (plane). More information would be useful here:

- what was/is above the exposed surface? ...more of the same, or a different bed?; ditto below.

- what does the bed look like in cross section? (I note your photo shows little obvious structure, but maybe that is due to the freshness of the broken surfaces)

- what do the laminae look like under the microscope?

- are the laminae continuous or discontinuous, in both 2D and 3D? (I use the term 'laminae' loosely, not knowing their real nature)